Acidic ocean robs coral of vital building material

CARBON dioxide has pillaged the Great Barrier Reef of a compound that corals and many sea creatures need to grow. The finding, from the first survey of ocean acidification around one of the world's greatest natural landmarks, supports fears that the ecosystem is on its last legs.

Bizarrely, the reef doesn't appear to be suffering from the effects of ocean acidification just yet. But that may be because it is balanced on a knife-edge between health and decay.

Oceans become acidic when they absorb CO2 from the atmosphere. Once dissolved, the gas reacts with carbonate to form bicarbonate, stripping seawater of the compound that many marine organisms including coral, shrimp and crabs need to build their shells or skeletons.

Bronte Tilbrook at CSIRO in Hobart, Tasmania, Australia, measured the concentration of aragonite - a form of calcium carbonate used by some creatures to build shells - at over 200 locations on the reef.

Corals grow well when the amount of aragonite in the water has a saturation level of 4.5. Below that, coral growth declines. Models suggest that if seawater becomes too low in aragonite, organisms with aragonite shells will dissolve. Studies in the Red Sea have found that some species of coral start to dissolve at a saturation of 2.8.

"Almost every bit of water we sampled was below 3.5," says Tilbrook, who presented his findings at Greenhouse 2011 in Cairns this week. Close to the shore, to the south of the reef, the saturation was 3.

Despite suboptimal conditions Tilbrook found little evidence that coral in the reef had reached their critical point. "They are still growing," he says. This could either mean that corals are more resilient than we thought, or they are on a knife-edge.

Will Howard of the Antarctic Climate and Ecosystems Cooperative Research Centre in Hobart has shown that some species of coral have a similar sensitivity to acidification as foraminifera in parts of the Southern Ocean, which are struggling to build their shells.

Howard and Tilbrook say this suggests the critical point for the corals of the Great Barrier Reef may be imminent. It is difficult to draw definite conclusions, however, because some 350 coral species are thought to inhabit the reef - with only a handful being tested under lab conditions.

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